The present invention relates generally to fluorescent X-ray film thickness gauges, and more particularly to fluorescent X-ray film thickness gauges having means for irradiating X-rays along an X-ray axis to a precise spot on a sample and for enabling an observer to view along the X-ray axis the spot on the sample at which the X-rays are to be directed.
In recent years, there has been an increasing demand for an accurate and reliable technique for measuring the thickness of film coatings in the electronic industry. For example, the need for such film thickness measurements is commonplace in the fabrication of integrated circuits, electric connections, printed circuit boards and in numerous other applications where a film coating of one material is applied on a base material. In general, the film coating is plated, coated, evaporated, sputtered or otherwise deposited on the base material, and with the highly developed fabrication techniques in use today, the film coatings are smaller in area, and therefore more difficult to measure, than ever before.
It is known in the art to use fluorescent X-ray film thickness gauges for measuring film thicknesses and such gauges are advantageous since they perform non-contact and nondestructive measurement. The conventional type fluorescent X-ray film thickness gauge is shown in FIGS. 1-2 in conjunction with a sample 1 having a film whose thickness is to be measured. The gauge comprises an X-ray tube 2 positioned above the sample 1 for irradiating X-rays along an X-ray axis 3 to a measuring point P on the sample and in response to such irradiation, the sample 1 emits fluorescent X-rays 4 from the measuring point P. A detector 5 is positioned to receive the fluorescent X-rays 4 emitted by the sample 1 for developing an output signal proportional to the amount of the fluorescent X-rays for use in measuring the film thickness. As known in the art, the principle of fluorescent X-ray measurement is based on the fact that when a material is irradiated by X-rays, fluorescent X-rays are emitted having wavelengths or energies characteristic of the elements contained in the material.
As shown in FIG. 1, in order to determine the point P on the sample 1 which is to be irradiated by X-rays, viewing means 6, which typically comprises a microscope and projector, is positioned above the sample so that an observer may view along an optical axis 7 the spot on the sample to be irradiated. However, since the optical axis 7 of the viewing means 6 does not coincide with the X-ray axis 3 of the X-ray tube 2, the shape of the target spot on the sample 1 which is irradiated by the X-rays differs from the shape of the target spot as viewed by the viewing means. In other words, since the surface of the sample 1 is not irradiated by the X-ray beam in a direction normal to the surface but rather is irradiated at an inclined angle relative to the normal, the shape of the irradiating point of the X-ray beam on the sample 1 is elliptical rather than circular as shown by the dotted line representation of the irradiating point, on an enlarged scale, in FIG. 2.
This discrepancy between the shape of the target spot as viewed by the observer through the viewing means 6 and as irradiated by the X-ray tube 2 can lead to measurement errors, particularly when measuring the film thickness of very small coating areas and/or of precisely shaped components such as are oftentimes found on integrated circuits. Since the target spot to be irradiated is determined by viewing through the viewing means 6 which presents to the observer a circular target area, and since the spot actually irradiated by the X-ray tube 2 is an elliptical area, the neighboring portions of the target area are also irradiated thereby leading to the likelihood of inaccurate measurements. This is so even if the X-ray beam is made as thin as possible since in all cases, the irradiating point will be elliptical rather than circular due to the inclination of the X-ray axis 3 relative to the optical axis 7. Another drawback of the conventional gauge is that if the sample 1 is moved vertically upward during measuring, the shape of the irradiating point becomes even more elliptical and the measuring accuracy becomes correspondingly less accurate.